This invention relates to a temperature-compensated constant voltage generating circuit.
A conventional circuit of this type is as shown in FIG. 1. In FIG. 1, reference numeral 1 designates a resistor; 2, a series circuit of m diodes; 3, a resistor; and 4, a voltage supply terminal. These elements 1, 2, 3 and 4 provide a voltage level V.sub.1. Further in FIG. 1, reference numeral 5 designates a level down circuit for shifting down the voltage level V.sub.1 by a voltage which is represented by the sum of n (n being an integer) times the base-emitter voltage of a transistor or the anode-cathode voltage of a diode, i.e., a p-n junction voltage, and a predetermined voltage; reference numeral 6 designates the input terminal of the circuit 5; reference numeral 7 designates the output terminal of the circuit 5; and reference character V.sub.2 designates the voltage level at the output terminal 7. One example of the aforementioned level down circuit is as shown in FIG. 2. In FIG. 2, reference numerals 4, 6 and 7 designate elements denoted by the same reference numerals in FIG. 1; 21, 22 and 23 are NPN transistors; 24 is a diode; 25 is a resistor; and 26, 27 and 28 are current sources. With n=4, the voltage drop across the resistor 25 corresponds to the above-described predetermined voltage.
The operation of the circuit will now be described.
The voltage levels V.sub.1 and V.sub.2 are represented by the following expressions (1) and (2) respectively: ##EQU1##
where V.sub.BE is the base-emitter voltage of the transistor or the anode-cathode voltage of the diode, R.sub.1 is the resistance of the resistor 1, R.sub.2 is the resistance of the resistor 3, V.sub.cc is the supply voltage, and V.sub.O is the voltage drop across the resistor 25.
If A is inserted for R.sub.1 /R.sub.2 is expression (2), then expression (2) can be rewritten as follows: ##EQU2## If the values V.sub.cc, V.sub.O and R.sub.1 /R.sub.2 are constant irrespective of temperature variation, then the second term in expression (3) is constant irrespective of any temperature variation. Therefore, in order to maintain V.sub.2 unchanged despite a temperature variation, the first term should be equal to zero. Therefore, the condition for making the value of V.sub.2 independent of temperature is: EQU m.multidot.A-n.multidot.(1+A)=0 (4)
If B is used in place of R.sub.2 /R.sub.1, expression (4) can be rewritten as follows: EQU m=n.multidot.(1+B) (5)
When expression (5) holds true, V.sub.2 is: ##EQU3##
Where the circuit shown in FIG. 1 is used practically, V.sub.2, V.sub.O, V.sub.cc and n are given so that B (=R.sub.2 /R.sub.1) and m are determined from expressions (5) and (6). In this case, the following two problems are involved:
1. The value m must be an integer. Therefore, as is apparent from expression (5), the variation of V.sub.2 due to temperature variation can be made zero only when n.multidot.R.sub.2 /R.sub.1 is an integer.
2. If, even when n.multidot.R.sub.2 /R.sub.1 is an integer, n or R.sub.2 /R.sub.1 is large, then m becomes considerably large. In practice, it is impossible to realize such a circuit.
In conclusion, it is, in general, impossible to make the variation of V.sub.2 due to temperature variation equal to zero with the circuit shown in FIG. 1.